The present invention relates in general to a visual inspection system suitable for gaining access within relatively small openings, and more particularly, to such a visual inspection system arranged in an underwater radioactive environment using fiberoptics for visually inspecting objects such as, for example, radioactive nuclear fuel assemblies.
In water cooled nuclear reactors, a plurality of elongated fuel elements and control element guide tubes are arranged, as a closely spaced array, in a unified structure known as a fuel assembly. The fuel elements are generally constructed from elongated cylindrical metal tubes containing nuclear fuel pellets and having both ends sealed by end plugs. These fuel assemblies are arranged in a vertically disposed lattice to form the reactor core. Typical reactor cores of this type are described and illustrated in U.S. Pat. No. 4,381,284.
Spent radioactive fuel assemblies are generally maintained underwater, for cooling and shielding purposes, during removal from a reactor and stored initially in a spent fuel pool. Although the fuel elements are designed to withstand the effects of the reactor operating environment, including those due to coolant hydraulics, reactor temperature and pressure, fission gas pressure, fuel expansion, and irradiation growth, defects in the fuel elements can occur which, if gone undetected, could permit the escape of radioactive fission products. It is, therefore, desirable and often necessary to inspect the fuel elements for such defects as cracks, reduction of thickness, deformation, impression and corrosion, so that corrective action, for example, replacement and/or repair of defective or failed fuel elements may be undertaken. On one hand, locating a defective fuel element within a fuel assembly is extremely difficult since such an assembly is radioactive and may contain hundreds of closely spaced fuel elements and guide tubes. On the other hand, disassembling and reassembling of these fuel assemblies is time consuming and may, in itself, result in fuel element damage. Thus, it can be appreciated that it is important to have a reliable system for locating defective fuel elements arranged in a closely spaced array and for the visual inspection thereof.
In reactors utilizing a liquid coolant, a number of devices and techniques have been proposed for locating individual defective fuel elements within the fuel assembly based upon detection and analysis of vibration, temperature differentials or ultrasonic phenomenon. These prior art detection devices and techniques have required, in general, that the fuel assemblies be at least partially disassembled as they do not permit access between the closely spaced fuel elements and guide tubes. Furthermore, these detection devices and techniques rely upon the dynamics of thermodynamic changes of state of the fluid which has leaked into the defective fuel element, typically boiling or condensation or both. One such prior art detection device which neither requires disassembling of the fuel assembly nor is dependent upon boiling or condensation of fluid within the fuel elements, is disclosed in U.S. Pat. No. 4,174,255. This detection device utilizes a transducer for emitting ultrasound in the megahertz frequency range for detecting water within the fuel element, indicative of a defective condition, or the presence of gas within the fuel element, indicative of a fuel element being free from any defects. However, such prior art detection device is not suitable for locating the specific defect, should one be present, nor for allowing the visual inspection of such a defect for identification of its type and severity.
Accordingly, it can be appreciated that there is an unsolved need for a radioactive fuel assembly inspection system adapted for locating defective fuel elements arranged in a closely spaced array, as well as permitting the visual inspection of such defects without the need of at least the partial disassembling of the fuel assembly.